Effect of the System’s Time Constants on the Dynamic Behavior of an Active Magnetic Bearing with an GaN-H-Bridge-Converter

Active magnetic bearings are inherently unstable systems requiring a highly dynamic control and a matching actuator. This paper presents a GaN-H-Bride converter with possible switching frequencies above 200kHz and investigates the effect of the three basic time constants or the respective frequencies in discrete control: the converter’s switching frequency, the measurement frequency, and the control frequency. Studying the switching behavior of the converter as well as the reference-value and disturbance response of the inner current and outer position control loops lead to three basic findings. Increasing the switching frequency without considering the required voltage and current in steady operation might impair the reference-value response. The optimum switching frequency depends on the minimum switch-on time. Increasing the measurement frequency has little effect on the reference-value response but significantly improves the disturbance response. The control frequency is the limiting factor for both inner and outer control loops in small to medium size active magnetic bearings.